This review summarizes current knowledge on mitochondrial disease biomarkers based on a literature review and discusses their clinical utility. It evaluates 13 biomarkers, including lactate, pyruvate, lactate:pyruvate ratio, creatine kinase, creatine, amino acid profiles, glutathione, malondialdehyde, GDF-15, FGF-21, gelsolin, neurofilament light-chain, and circulating cell-free mtDNA. Most biomarkers show mixed results depending on the study, especially when considering their utility for specific mitochondrial diseases versus general mitochondrial conditions. However, in large studies, GDF-15 and FGF-21 appear to have the greatest value, though they are not perfect. Additional studies are needed, particularly for newer biomarkers. Understanding these biomarkers is crucial for early detection, improved patient management, and developing targeted therapies. Lactate and pyruvate are frequently measured together, with the lactate:pyruvate ratio being a more specific biomarker. Creatine kinase and creatine are less useful due to their association with other conditions. Amino acid profiles can help identify certain mitochondrial diseases, but results are variable. Glutathione and malondialdehyde are not strong biomarkers. GDF-15 and FGF-21 are considered the most promising biomarkers for mitochondrial diseases, with GDF-15 showing slightly better results in most studies. Novel approaches like metabolomic and lipidomic testing are being explored, but more research is needed. Molecular testing remains the standard for diagnosis, while biochemical testing can be useful in unclear cases. The review highlights the complexity of mitochondrial diseases and the need for further research to improve diagnostic accuracy and treatment options.This review summarizes current knowledge on mitochondrial disease biomarkers based on a literature review and discusses their clinical utility. It evaluates 13 biomarkers, including lactate, pyruvate, lactate:pyruvate ratio, creatine kinase, creatine, amino acid profiles, glutathione, malondialdehyde, GDF-15, FGF-21, gelsolin, neurofilament light-chain, and circulating cell-free mtDNA. Most biomarkers show mixed results depending on the study, especially when considering their utility for specific mitochondrial diseases versus general mitochondrial conditions. However, in large studies, GDF-15 and FGF-21 appear to have the greatest value, though they are not perfect. Additional studies are needed, particularly for newer biomarkers. Understanding these biomarkers is crucial for early detection, improved patient management, and developing targeted therapies. Lactate and pyruvate are frequently measured together, with the lactate:pyruvate ratio being a more specific biomarker. Creatine kinase and creatine are less useful due to their association with other conditions. Amino acid profiles can help identify certain mitochondrial diseases, but results are variable. Glutathione and malondialdehyde are not strong biomarkers. GDF-15 and FGF-21 are considered the most promising biomarkers for mitochondrial diseases, with GDF-15 showing slightly better results in most studies. Novel approaches like metabolomic and lipidomic testing are being explored, but more research is needed. Molecular testing remains the standard for diagnosis, while biochemical testing can be useful in unclear cases. The review highlights the complexity of mitochondrial diseases and the need for further research to improve diagnostic accuracy and treatment options.